Microfluidic devices are typically formed of substrates (made of silicon, glass, ceramic, plastic and/or quartz) which include a network of micro-channels through which fluid flows under the control of a propulsion mechanism. The micro channels typically have at least one dimension which is on the order of nanometers to hundreds of microns.
Microfluidic devices process minute amounts of fluid sample to determine the physical and chemical properties of the sample. Microfluidic devices offer several advantages over a traditional macro-scale instrumentation. For example, in general, they require substantial smaller fluid samples, use far less reagent, and process these fluids at substantially greater speeds than macro-scale equipment.
In many cases, the accuracy of such fluid processing depends upon the relative amounts of sample and reagent used. For example, when a sample is analyzed for a DNA “fingerprint,” the results may depend upon the concentration of reagents used to amplify DNA present in the sample. Thus, if an improper ratio of sample to reagent is used, the result may be inaccurate. Because microfluidic devices process samples and reagents in minute amounts, even a small absolute uncertainty in the amount of reagent or sample used can introduce uncertainty to the results of a microfluidic analysis.
Variances in the amount of samples and reagents processed by a microfluidic device may originate from several sources. For example, some microfluidic devices manipulate continuous, flowing streams of liquid. Changes in the viscosity of the liquid can alter the flow rate of the streams and, correspondingly, the time required to introduce a predetermined amount of material to a given location of the microfluidic device. Sample dilution may occur where a liquid flow stream is used to move sample components from one location to another within a microfluidic device.
Microfluidic analysis of particles suspended in a fluid (such as cells within body fluids) is especially challenging due to the relatively small number of particles available for analysis and the inherent difficulty in manipulating such fluids.